E. Koray Akdogan

Piezoelectric and acoustic materials for transducer applications

Fundamentals of Piezoelectricity.- Thermodynamics of Ferroelectricity.- Piezoelectricity and Crystal Symmetry.- Crystal Chemistry of Piezoelectric Materials.- Piezoelectric and Acoustic Materials for Transducer Technology.- Lead-Based Piezoelectric Materials.- KNN-Based Piezoelectric Ceramics.- Bismuth-based Piezoelectric Ceramics.- Electropolymers for Mechatronics and Artificial Muscles.- Low-Attenuation Acoustic Silicone Lens for Medical Ultrasonic Array Probes.- Carbon-Fiber Composite Materials for Medical Transducers.- Transducer Design and Principles.- Piezoelectric Transducer Design for Medical Diagnosis and NDE.- Piezoelectric Transducer Designs for Sonar Applications.- Finite Element Analysis of Piezoelectric Transducers.- Piezoelectric Transducer Fabrication Methods.- Piezoelectric Fiber Composite Fabrication.- Composition Gradient Actuators.- Robocasting of Three-Dimensional Piezoelectric Structures.- Micropositioning.- Piezoelectric Actuator Designs.- Piezoelectric Energy Harvesting using Bulk Transducers.- Piezocomposite Ultrasonic Transducers for High-Frequency Wire Bonding of Semiconductor Packages.- Piezoelectric MEMS: Materials and Devices.- High-Frequency Ultrasonic Transducers and Arrays.- Micromachined Ultrasonic Transducers.

Tunable Dielectric Properties of BST Thin Films for RF/MW Passive Components

Heteroepitaxial Ba 0.6 Sr 0.4 TiO 3 films were deposited on (100) LaAlO 3 and (100) MgO substrates via pulsed laser deposition. Film thickness varied from 22 nm to 1.15 w m. All films were examined with x-ray diffraction and cross-section FESEM to determine both epitaxy and thickness, and stoichiometry was confirmed with RBS measurements. Microwave dielectric measurements were carried out in the range of 1-20 GHz, using an interdigitated electrode array. The state of strain in the films as a function of thickness and substrate type was correlated with the observed capacitive tunability. The tunability for both film series was shown to exhibit markedly different behavior depending on the type of strain, compressive or tensile. Maximum dielectric tunabilities of ∼ 65% were achieved for the thickest films in both film sets, however the tunability of the thinnest films were much higher for the films grown on MgO.

Metallic molybdenum disulfide nanosheet-based electrochemical actuators

Actuators that convert electrical energy to mechanical energy are useful in a wide variety of electromechanical systems and in robotics, with applications such as steerable catheters, adaptive wings for aircraft and drag-reducing wind turbines. Actuation systems can be based on various stimuli, such as heat, solvent adsorption/desorption, or electrochemical action (in systems such as carbon nanotube electrodes, graphite electrodes, polymer electrodes and metals). Here we demonstrate that the dynamic expansion and contraction of electrode films formed by restacking chemically exfoliated nanosheets of two-dimensional metallic molybdenum disulfide (MoS2) on thin plastic substrates can generate substantial mechanical forces. These films are capable of lifting masses that are more than 150 times that of the electrode over several millimetres and for hundreds of cycles. Specifically, the MoS2 films are able to generate mechanical stresses of about 17 megapascals—higher than mammalian muscle (about 0.3 megapascals) and comparable to ceramic piezoelectric actuators (about 40 megapascals)—and strains of about 0.6 per cent, operating at frequencies up to 1 hertz. The actuation performance is attributed to the high electrical conductivity of the metallic 1T phase of MoS2 nanosheets, the elastic modulus of restacked MoS2 layers (2 to 4 gigapascals) and fast proton diffusion between the nanosheets. These results could lead to new electrochemical actuators for high-strain and high-frequency applications.

Phenomenological theory of size effects on the cubic-tetragonal phase transition in BaTiO3 nanocrystals

We study the first order tetragonal→cubic ferrodistortive phase transition in single domain mechanically unconstrained BaTiO3 nanocrystals with no depolarization fields. We demonstrate that Landau coefficients up to the sixth order as a function of particle size are needed to describe the nanoparticle size induced changes in tetragonal phase stability as well as dielectric properties in the vicinity of the phase transition. The thermodynamic potential developed in this work is self-consistent, fully commensurate polar point group 4 mm, and is able to predict the size-induced phase transition, as well as the metastable tetragonal phase in the cubic phase field. We also evaluate the changes in single domain static dielectric susceptibilities demonstrate significant deviations from bulk properties. Specifically, we find a decrease in dielectric susceptibility at the transition temperature with decreasing particle size, which we verify and find to be in agreement with the predictions from lattice dynamics considerations.

Barium strontium titanate thin film varactors on r-plane sapphire

This paper presents the microwave properties of barium strontium titanate (BST) thin films on r-plane sapphire substrates. A series of films with thickness 25-400 nm was prepared by pulsed laser deposition (PLD). Microwave properties of the films, including capacitance tunability and loss tangent, were extracted by patterning interdigitated capacitors (IDCs) on the film surface. The highest tunability of 64% was observed in the 200 nm film. These results demonstrate the possibility of integrating BST into the silicon on sapphire process.

Thickness and Strain Effects on RF/Microwave Properties of BST Thin Films on NdGaO 3 Substrates

Ba 0.60 Sr 0.40 TiO 3 thin films were deposited on oriented NdGaO 3 substrates by pulsed-laser deposition. Film thickness ranged from 20 nm to 800 nm. Microstructural features, as evaluated with AFM and FESEM, have exhibited high quality thickness dependent topography. X-ray analyses have shown consistently textured films of high crystallinity. Permittivity, Q-factor, and tunability, were investigated using interdigitated capacitors in the 0.1–20 GHz range. Effect of film thickness on dielectric properties and tunability in polycrystalline textured films exhibited strong thickness dependence in their elasto-dielectric properties. Tunability up to 45% was observed at moderate field levels (∼ 7 MV/m), while the Q-factors remained ≤40.